1. Field of the Invention
The present invention relates to an ozone generator for use in an industrial field requiring a large amount of ozone such as drinking water and sewage treatment, pulp bleaching, and the like.
2. Description of Related Art
As an industrial scale ozone generator described above, an ozone generator for ozonizing an oxygen-containing feed gas by using silent discharge is known to the public in Japanese Patent Application Laid-open No. 2-184506 (U.S. Pat. No. 5,034,198) and the disclosure of this patent is incorporated by reference in the present specification.
Construction of the above-described conventional ozone generator is shown in FIGS. 25 to 27. In FIGS. 25A and 25B, a vessel of the ozone generator basically comprises a cylindrical body 1 made of stainless steel or the like which is highly corrosion resistant to ozone, end plates 2 and 3 for closing both end openings of the body 1, and a pair of circular partition walls 5 and 6 disposed with a space to form a water jacket 4 surrounding a ground electrode which will be described later at the center in the body. In the periphery of the body 1, a feed gas inlet 7 is provided between the end plate 2 and the partition wall 5, an ozonized gas outlet 8 is provided between the other end plate 3 and the partition wall 6, and a cooling water inlet 9 for supplying cooling water from the outside through the water jacket 4 formed between the partition walls 5 and 6 and a cooling water outlet 10 are provided.
Further, inside the body 1, there are provided a plurality of sets of ozone generating tubes each set consisting of a tubular cylindrical ground electrode 11 made of ozone resistant stainless steel and a high voltage electrode 13 concentrically disposed with respect to the ground electrode 11 so as to form a discharge gap there between, the plurality of sets of ozone generating tubes extending over between the partition walls 5 and 6 and horizontally disposed penetrating the partition walls. The high voltage electrode 13 is connected to a high voltage terminal of a high frequency power source 16 by a bus bar 14 arranged inside the body 1 through a bushing 15 provided in the body 1. Further, the ground side of the high frequency power source 16 is grounded together with the body 1.
Still further, connections between the body 1 and the partition walls 5 and 6, and those between the partition walls 5 and 6 and the ground electrode 11 penetrating the partition walls are liquid-tight seam welded to form an integral construction. The end plates 2 and 3 are retained on the opposite ends of the body 1 with screws or the like through an air-tight sealing member 17 such as a flat plate-formed gasket or an O-ring.
Next, a detailed structure of the conventional ozone generating tube is shown in FIGS. 26A and 26B. Specifically, the high voltage electrode 13 concentrically disposed in the cylindrical tube ground electrode 11 comprises a round-sectioned glass tube 13a with one end portion closed and a metal film 13b formed by sputtering on the inner peripheral surface of the glass tube 13a, thereby forming a nearly uniform discharge gap 12 over the entire length of the electrode. Further, the bus bar 14 leading into the chamber and the metal film 13b are connected through a conductive contactor 13c. Cooling water flowing into the water jacket 4 uses ordinary water and is recirculated via an external recirculation pump and a cooling heat exchanger (both not shown).
With the above-described arrangement, the oxygen-containing feed gas (oxygen or air, previously demoistured and introduced into a gas chamber) supplied to a feed gas chamber from the gas inlet 7 flows in the discharge gap 12 along the ozone generating tubes incorporated in the body. In this state, when AC voltage is applied between the ground electrode 11 and the high voltage electrode 13 from the high frequency power source 16, oxygen in the feed gas is ozonized by silent discharge produced in the discharge gap, an ozonized gas flows out to an ozonized gas chamber in the body, and is then fed to an ozone user through the gas outlet. Heat evolved in association with the ozone production (ozone generation reaction is an exothermic reaction) is transmitted to the cooling water flowing in the water jacket 4 and removed out from the system. The gas outlet 8 is provided with an exhaust valve (not shown) for gas pressure adjustment, thereby adjusting the inner pressure of the gas chamber to a positive pressure relative to the atmospheric pressure. In an actual product, one unit of ozone generator incorporates several tens to several hundreds of ozone generating tubes disposed in parallel.
FIG. 27 is a schematic illustration showing a layout of a main unit, a power source, and cooling system accessory devices in a prior art ozone generator, in which the main unit (vessel) 18 of the ozone generator is connected with external cooling water piping 19 as a cooling water supply means, and cooling water recirculates via a recirculation pump 20 and a water-cooled heat exchanger 21. Furthermore, a high frequency power source 16 comprises a combination of an inverter 16a for converting commercial power into high frequency power with a step-up transformer 16b and is connected to supply power to the main unit 18 through a high voltage cable 22.
However, the above-described prior art ozone generator has problems as will be described below.
1) When, with the aim of enhancing the concentration of ozone produced, power density (discharge power per unit discharge area) supplied to the discharge electrodes is increased, charged particles in the plasma collide with the electrodes to heat the electrodes, decomposing the produced ozone (the decomposition reaction of ozone is an endothermic reaction, and decomposition is thus accelerated as the temperature increases). Therefore, the electrode portion must be cooled efficiently even further. However, in the prior art electrode cooling structure, only the ground electrode is cooled from the outer peripheral surface side by flowing water in the water jacket, but the high voltage electrode opposing across the discharge gap is not directly water-cooled.
On the other hand, considering heat evolution at the discharge gap between electrodes, since a creeping streamer is formed on the surface of the dielectric (glass tube) in association with silent discharge, a substantial amount of heat evolution occurs on the surface of the dielectric in addition to the heat evolution at the discharge gap.
Therefore, since, with the prior art electrode cooling structure, the discharge gap acts as a kind of heat insulation layer, heat evolved on the high voltage electrode side cannot be efficiently removed, resulting in reduced ozone generation efficiency and concentration of ozone produced.
2) To improve the ozone generation efficiency, it is important that a discharge gap is uniformly formed which is optimum for maintaining stable silent discharge between the ground electrode and the high voltage electrode. However, since in the prior art structure, the glass tube as a dielectric is inserted directly into the tube of the ground electrode, uniformity of the discharge gap along the longitudinal direction is limited by dimensional precision of the glass tube, and it has been very difficult to set the discharge gap equal to or less than 2 mm in an actual product.
3) Since, in the prior art structure, the ground electrode is integrally welded to the partition walls of the gas chamber, it is very difficult to inspect and replace the ground electrode.
4) Furthermore, since the glass tube as a dielectric is disposed outside the high voltage electrode, there is a danger of damaging the glass tube by hitting against other structures when the high voltage electrode is removed for maintenance or inspection. Still further, the metal film (electrode portion) sputtered onto the inner surface of the glass tube tends to peel during repetitions of heat cycle of operation and stop of the ozone generator due to a difference in thermal expansion coefficient between the glass tube and the metal film.